1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing it, and more particularly to a chip size package and a method for manufacturing it. The chip size package is also referred to as xe2x80x9cCSPxe2x80x9d which generally means a package having a size equal or slightly larger than the size of a chip. The chip size package intends to realize high density packaging. The present invention is directed to exposure of the head of a metal post to be adopted in the CSP.
2. Related Art
The structures known previously in this technical field are a xe2x80x9cBGAxe2x80x9d (Ball Grid Array) having a plurality of solder balls arranged in plane; a xe2x80x9cfine pitch BGAxe2x80x9d in which the ball pitch in BGA is further reduced so that the outer shape of a package has a size approximately equal of the chip size; etc.
In recent years, a xe2x80x9cwafer CSPxe2x80x9d has been disclosed in xe2x80x9cNIKKEI MICRODEVICExe2x80x9d August 1998, pp. 44-71. The wafer CSP is basically a CSP in which wirings and pads in an array are previously formed in a wafer process before a chip is diced. It is expected that this technique integrates the wafer process and package process to reduce the package cost greatly.
The wafer CSP is classified into two types of a sealing resin type and a re-wiring type. The sealing resin type is a structure in which the surface is covered with sealing resin as in a conventional package. Specifically, in the sealing type structure, a metal post is formed so as to project on a wiring layer of the chip surface, and its periphery is sealed by sealing resin.
Generally, if a package is mounted on a printed board, stress produced owing to a difference in a thermal expansion coefficient between the package and printed board is concentrated on the metal post. It is known that as the metal post is longer in the resin sealing type, the stress is dispersed more greatly.
On the other hand, the re-wiring type is a structure in which xe2x80x9cre-wiringxe2x80x9d is formed so that it is connected to a bonding pad of a semiconductor surface (without using the sealing resin) as shown in FIG. 10. In the re-wiring structure, an Al (aluminum) electrode 52, a wiring layer 53 and an insulating layer 54 are stacked on the surface of a chip 51. A metal post 55 is formed on the wiring layer 53. A solder ball 56 is formed thereon. The wiring layer 53 is used as the re-wiring means for aligning/arranging the solder bump 53 in a prescribed array on the chip 51.
The sealing type of the wafer CSP, in which the metal post lengthened to 100 xcexcm is reinforced by sealing resin, can acquire great reliability. However, the process of forming sealing resin, which must be carried out using molding die in a post-step, is complicate.
On the other hand, the re-winding type has an advantage that its process is comparatively simple and almost all the steps can be carried out in a wafer process. However, it is necessary to relax the above stress to enhance the reliability.
As clearly seen from FIG. 11 in which the wiring layer 53 in FIG. 10 is omitted, an opening with an Al electrode 52 exposed is formed and at least one layer of a barrier metal 58 is formed between the metal post 55 and Al electrode 52. The solder ball 56 is formed on the metal post 55.
In the case of FIG. 20, polyimide resin is applied to cover the metal post entirely, and after having been set or hardened, its upper face is ground to expose the head of the metal post. However, it is very difficult to control this grinding step so that evenness in the soldering nature and height of the solder ball 56 may be deteriorated.
This can be seen more clearly from FIG. 9. In the case that after the metal post is formed, a thick dielectric resin layer is applied to cover the head thereof, if the surface is ground after the resin layer has been set, because of unevenness of the wafer surface and contact face of a grinder, some metal posts have a barrier metal scraped in its head, and some metal posts have a head with the dielectric resin layer being left. In the latter, the head of the metal post cannot be exposed so that the solder ball or solder bump cannot be formed.
The present invention has been accomplished in order to solve the above problem.
An object of the present invention is to provide a semiconductor device which can provide a sure contact, can be easily manufactured and acquire great reliability.
The first aspect of the present invention solves the above problem by provision of a dielectric layer of thermosetting resin which covers the chip surface inclusive of said wiring layer and is located, in its main surface on the periphery of said metal post, at a lower position than a head of said metal post due to thermal shrinkage occurring during thermal setting.
Generally, thermosetting resin is shrunk owing to its thermal setting. The resin to be adopted in the present invention should have a very high shrinkage rate to reduce its thickness or height greatly. The resin is baked for setting so that the surface thereof can be located at a lower position than the head of the metal post as shown in FIG. 8. Thus, the heads of all the metal posts can be exposed. All the solder balls can be fixed on the metal posts in good contact therewith.
The second aspect of the present invention solves the problem in such a manner that resin which is mainly composed of amic acid which is used as the resin suffering great shrinkage is applied and shrunk.
The amic acid has a very high shrinking rate of 30-50% and shrinks at a low shrinking temperature of 300-500xc2x0 C. Therefore, by using this amic acid as the dielectric layer, it is possible to cover the chip surface and maintain the contact with the metal post.
The third aspect of the present invention solves the problem by implementing the steps of forming a second dielectric layer of resin suffering thermal shrinkage during thermal setting on the chip surface on which said first dielectric layer, said wiring layer and said metal post are formed; and shrinking said second dielectric layer by heat treatment so that its main surface on the periphery of said metal post is located at a position lower than a head of said metal post; and forming the solder ball on said metal post.
In this configuration, the second dielectric layer is held covering the chip surface and its surface is lowered owing to its thermal shrinkage so that the contact can be made easily and surely with great reliability.
The fourth aspect of the present invention solves the problem by adopting resin of an amic acid of an applying type or a film type.
The fifth aspect of the present invention solves the problem in such a manner that said film is equipped, on its surface, with a sheet of a different material from said dielectric layers, and the sheet is peeled off to expose said metal post.
The present invention has been accomplished in order to solve the above problem.
First, a dielectric layer of thermosetting resin is adopted in which its main surface on the periphery of said metal post can be located at a lower level than a head of said metal so that a barrier layer can be easily formed on the metal post exposed from the dielectric resin layer.
The film thickness and shape of the barrier layer can be provided with high accuracy and good reproducibility, and hence the reliability of the solder ball or solder bump can be improved.
Secondly, the resin suffering a great reduction of the film thickness owing to thermal shrinkage occurring during thermal setting is adopted so that the head of the metal post can be exposed from the resin and a barrier layer can also be formed on the exposed portion.
Thirdly, the metal post is made of Cu so that an oxidation resistant film covering the exposed portion of the metal post prevents oxidation thereof. This also prevents reduction of the mechanical strength which will occurs when soldering is made with copper oxide being left on the surface of the metal post.
Fourthly, the oxidation resistant film is composed of an underlying layer of Ni and an overlying layer of Au so that Ni prevents oxidation of the metal post, and Au improves its contact and prevents oxidation of Ni.
Fifthly, the dielectric resin layer of amic acid is adopted so that its great reduction rate of the film thickness can expose the head of the metal post effectively.
Sixthly, the manufacturing process comprises the steps of:
forming a Cu wiring layer,
forming a metal post on said wiring layer;
covering the chip surface inclusive of the metal post with a dielectric resin layer;
shrinking the dielectric layer by heat treatment to make its main surface on the periphery of said metal post lower than a head of said metal post so that the head of the metal post is exposed;
forming a barrier layer on the exposed metal post; and
forming a solder ball of said metal post.
Seventhly, the dielectric layer is an applying type or film type material which is made of amic acid and can be located lower than the head of the metal post by heat treatment.
Particularly, the film type is pressurized so that the sheet is kept in contact with the head of the metal post. Therefore, by peeling off the sheet after having been set, the head of the metal post can be easily exposed.